1. Field of the Invention
The present invention relates to a suck back valve in which liquid dripping, for example occurring at a supply port of a fluid passage, is prevented by sucking a predetermined amount of fluid flowing through the fluid passage by displacement of a diaphragm, yet wherein the sucked amount of fluid can be stabilized.
2. Description of the Related Art
There has hitherto been known, for example, a semiconductor wafer manufacturing process in which a suck back valve is employed. In the suck back valve, when supply of coating liquid to the semiconductor wafer is halted, there is a function for preventing so called liquid drip, wherein minute amounts of coating liquid drip from a supply port onto the semiconductor wafer.
The suck back valve according to such a conventional technique is shown in FIG. 7 and is disclosed, for example, by Japanese Utility Model Publication No. 8-10399.
Such a suck back valve 1 comprises a main valve body 5 formed with a fluid passage 4 interconnecting a fluid inlet port 2 and a fluid outlet port 3, and includes a bonnet 6 connected to an upper portion of the main valve body 5. A diaphragm 7 made up of a thick-walled portion and a thin-walled portion is disposed in the center of the fluid passage 4. A pressurized fluid supply source 13 is connected to the bonnet 6, and a pressurized fluid supply port 8, for supplying pressurized air for operating the diaphragm under a switch-over action of a directional control valve, is formed in the bonnet 6.
A piston 9 is fitted to the diaphragm 7, with a v-packing 10 installed in the piston 9 and slidable along an inner wall surface of the main valve body 5 and serving a sealing function. Further, a spring 11 which normally presses the piston in a upward direction, is disposed in the main valve body 5.
An adjusting screw 12 is disposed on the top of the bonnet 6 which, by increasing or decreasing a screw-in amount thereof, abuts against the piston 9 and adjusts a displacement amount of the piston 9, whereby the amount of coating liquid sucked by the diaphragm 7 is adjusted.
A coating liquid supply source 14, storing therein a coating liquid, is connected to the pressurized fluid inlet port 2 through a conduit 15 such as a tube, and further, between the coating liquid supply source 14 and the fluid inlet port 2, an ON/OFF valve 16, which is constructed separately from the suck back valve, is connected. The ON/OFF valve 16 performs a function of switching between a supply state, and a supply-halted state, of the coating liquid to the suck back valve 1, by energization and de-energization of the ON/OFF valve 16.
Operation of the suck back valve 1 shall now be explained in outline. In an ordinary state in which the fluid is supplied from the fluid-inlet port 2 to the fluid-outlet port 3, the piston 9 and diaphragm 7 are displaced downwardly in an integrated manner in accordance with action of the pressurized fluid supplied from the pressurized fluid supply port 8. The diaphragm 7, which is coupled to the piston 9, protrudes into the fluid passage 4 as shown in FIG. 7 by the two-dot-chain line.
At that point, in the case that the flow of coating liquid inside the fluid passage 4 is halted by a switch-over action of the ON/OFF valve 16, by halting the supply of pressurized air from the pressurized fluid supply port 8, the piston 9 and diaphragm 7 are raised in unison under action of the elastic force of the spring 11, wherein coating liquid remaining in the fluid passage 4 is sucked by a negative pressure action of the diaphragm 7, and liquid dripping at an unillustrated supply port is prevented.
Incidentally, in the suck back valve 1 according to the conventional technique, in order to adjust with high accuracy the flow amount of pressurized air supplied from the pressurized fluid supply source 13, a fluid pressure control device 18 is interveningly installed between the pressurized fluid supply source 13 and the pressurized fluid supply port 8 through a conduit 17 such as a tube. The fluid pressure control device 18 is formed separately and apart from the suck back valve 1, wherein an unillustrated control element comprising for example an electrostrictive element for converting an electric signal to air pressure, is arranged in the fluid pressure control device 18.
Notwithstanding, in the suck back valve 1 according to the conventional technique, because reproducibility of the displacement with respect to voltage applied by the electrostrictive element, which possesses hysteresis, is rather poor, it becomes troublesome to control with high accuracy the flow amount of pressurized air which is supplied to the pressurized fluid supply port 8. Further, such an electrostrictive element experiences large clipping, along with the disadvantage of inferior durability.
Further, in the case of the suck back valve 1 according to the conventional technique, pipe connecting operations between the suck back valve 1 and the fluid pressure control device 18, as well as between the suck back valve 1 and the ON/OFF valve 16, become necessary, adding complexity to the system, and further, because the fluid pressure control device 18 and the ON/OFF valve 16 respectively are installed externally of the suck back valve 1, dedicated space therefor is needed, with the disadvantage that installation space increases.
Furthermore, fluid passage resistance is increased by the piping which is connected between the suck back valve 1 and the fluid pressure control device 18, with the disadvantage that response accuracy (responsiveness) of the diaphragm 7 is deteriorated.
Still further, a driving device 19 is specially required for switching the ON/OFF valve 16 between ON and OFF states, wherein along with the added complexity of pipe connecting operations between the ON/OFF valve 16 and the driving device 19, there is the drawback of soaring production costs.